1. Field of the Invention
The present invention relates to a tool holder to be removably attached to a spindle of a machine tool, as well as to a tool holder attachment mechanism.
2. Description of Related Art
A conventional tool holder has a structure in which a shank portion is tapered such that diameter decreases rearward (herein, the term xe2x80x9crearxe2x80x9d or xe2x80x9crearwardxe2x80x9d refers to a direction toward a machine tool, and the term xe2x80x9cfrontxe2x80x9d or xe2x80x9cfrontwardxe2x80x9d refers to a direction away from the machine tool). A taper hole is formed in a front-end portion of a spindle of a machine tool such that diameter decreases rearward. The shank portion of the tool holder is inserted into the taper hole and drawn rearward by means of a draw member, whereby the tool holder is fixedly attached to the spindle.
However, the shank portion is restrained only through contact between the tapered outer circumferential surface of the shank portion and the wall surface of the taper hole formed in the spindle, while a gap is maintained between the front-end face of the spindle and the rear-end face of a flange portion of the tool holder located at the front side of the shank portion. Accordingly, the shank portion cannot be held with a sufficiently large force for coping with high-speed cutting or low-speed, heavy-duty cutting.
In an attempt to solve the problem, a force for restraining the shank portion of the tool holder has been increased. Specifically, restraint is effected through abutment between the front-end face of the spindle and the rear-end face of the flange portion of the tool holder, in addition to the restraint effected by means of the tapered outer circumferential surface of the shank portion and the wall surface of the taper hole.
However, in order to effect such restraint, relevant manufacturing tolerance must be rendered more severe, causing an increase in cost. Also, after long-term use of the spindle, need to repolish the taper hole arises. Since the size of the taper hole increases as a result of repolishing, restraint of the tool holder is effected simply by means of the end face of the flange portion of the tool holder and that of the spindle. Since no contact is established between the tapered shank portion and the taper hole, a tool shakes, disabling continuation of machining. Also, when the spindle rotates at high speed, a front-end portion of the taper hole formed in the spindle expands as a result of action of a centrifugal force, causing reduction in restraint effected by the taper hole. As a result, a tool held by the tool holder vibrates, causing impairment in machining accuracy.
In order to cope with the problem, a shank portion of a holder body of a flanged tool holder is formed to have a straight portion, and a taper cone is fitted to the straight portion. The taper cone is oriented such that diameter decreases rearward. The taper cone is preloaded by means of Belleville springs (disc springs) which are interposed between the taper cone and the shank portion, such that the outer circumferential surface of the taper cone is pressed against the wall surface of the taper hole. Application of the preload and rearward drawing of the holder body solve the problem to a certain extent, and manufacture is relatively easy. However, a force for drawing the holder body must be increased; otherwise, the taper cone is not sufficiently restrained by means of the wall surface of the taper hole.
In another tool holder, a shank portion of a holder body includes a straight larger-diameter end portion and a straight smaller-diameter end portion. A taper cone includes a corresponding larger-diameter straight hole and a corresponding smaller-diameter straight hole. However, as a result of employment of the straight holes in the taper cone, the holder body must be drawn by a considerably large force in order to effect sufficient restraint of the tool holder.
FIG. 17 shows a tool holder 30 which is fixedly attached to a tool machine, such as a machining center, by means of a conventional tool holder attachment mechanism. As shown in FIG. 17, the tool holder 30 includes a taper shank portion 31; a flange portion 32 to be gripped located at the larger-diameter side of the taper shank portion 31; and a cylindrical tool attachment portion 33 extending from an end face of the flange portion 32 in opposition to and in alignment with the taper shank 31.
In attachment of the tool holder 30 to a spindle 40 of a machine tool, the taper shank portion 31 is inserted into a taper hole 41 formed in the spindle 40, and a pull-stud draw mechanism disposed within the spindle 40 is engaged with an inserted end of the taper shank portion 31. The inserted end of the taper shank portion 31 is then drawn rearward by means of the pull-stud draw mechanism so as to closely fit the taper shank portion 31 into the taper hole 41, thereby attaching the tool holder 30 to the spindle 40.
According to JIS or ISO standard, in order to closely fit the taper shank portion 31 of the tool holder 30 into the taper hole 41 formed in the spindle 40, a predetermined gap D (about 2 mm or 3 mm) must be provided between an end face 401 of the spindle 40 and a corresponding end face 311 of the flange portion 32 of the tool holder 30. The standard prescribes that the tolerance in manufacture of the taper shank portion 31 with respect to the gap D be xc2x10.4 mm.
Through establishment of the gap D between the end face 401 of the spindle 40 and the end face 311 of the flange portion 32 of the tool holder 30, the taper shank portion 31 of the tool holder 30 can be closely fitted into the taper hole 41 formed in the spindle 40. However, close contact cannot be established between the end face 311 of the flange portion 32 of the tool holder 30 and the end face 401 of the spindle 40. As a result, a cutting load is concentratedly imposed on the taper shank portion 31 of the tool holder 30, accelerating tendency toward fretting corrosion of the contact surface between the taper hole 41 and the taper shank portion 31. Further, coupling rigidity between the tool holder 30 and the spindle 40 is impaired, resulting in a failure to perform heavy-duty cutting.
In order to cope with the problem, Japanese Utility Model Application Laid-Open (kokai) No. 15947/1994 proposes a tool holder attachment-mechanism in which a pair of horseshoe-like spacers 34 (FIG. 17) are screwed on the end face of a flange portion of a tool holder so as to fill the gap between the end face of a spindle and that of the flange portion.
In the conventional tool holder attachment mechanism shown in FIG. 17, the end face 311 of the flange portion 32 of the tool holder 30 can be brought into close contact with the end face 401 of the spindle 40 via the spacer 34, which is screwed on the end face 311 of the flange portion 32 of the tool holder 30. As a result, high coupling rigidity is established between the tool holder 30 and the spindle 40, thereby enabling heavy-duty cutting. However, since the spacer 34 having a thickness corresponding to the gap D must be custom-made for each model of tool holder, manufacture and management of the spacers 34 are troublesome and involve increased cost. The spacers 34 may be attached to the end face of the spindle. However, this causes the following problem. In the case of a newly manufactured machine tool, threaded holes can be formed in the end face of a spindle for use in screwing the spacers 34 into place. However, in the case of an existing machine tool, cutting such threaded holes raises a problem in machining accuracy, since, among components of the machine tool, the spindle involves the highest required accuracy.
The present invention has been accomplished in view of the above-mentioned problems. A first object of the present invention is to provide a tool holder which can be fitted into a taper hole formed in a spindle even when a force for drawing a holder body is relatively small, to thereby enable high-speed cutting or low-speed, heavy-duty cutting. Specifically, the first object is to provide a tool holder in which the outer circumferential surface of a shank portion of the holder body and the inner circumferential surface of a taper conexe2x80x94which is fitted to the shank portionxe2x80x94are tapered more gently than are the wall surface of the taper hole and the outer circumferential surface of the taper cone (the taper hole and the taper cone are tapered such that diameter decreases rearward), so that the outer circumferential surface of the shank portion is locked into the taper cone through so-called wedging effect, and the outer circumferential surface of the taper cone imposes a radially expanding force on the wall surface of the taper hole.
A second object of the present invention is to provide a tool holder attachment mechanism allowing application of a single spacer to every tool holder or allowing easy attachment of the spacer to an either new or existing machine tool. Another object is to provide a highly safe tool holder attachment mechanism in which the spacer assumes an annular shape so as not to fly out during high-speed rotation.
A further object of the present invention is to provide a tool holder attachment mechanism in which a spacer bonded to the end face of a spindle permits use of a tool holder involving a problem in that the gap between the end face of the spindle and the and face of a flange portion of the tool holder becomes smaller than a gap specified in an industrial standard.
To achieve the first object, the present invention provides a tool holder to be removably fitted into a taper hole which is formed in a spindle of a machine tool such that diameter decreases rearward, comprising: a holder body, a rear portion of the holder body being formed into a shank portion, a flange portion being formed at a front side of the shank portion; a taper cone fitted to the shank portion and tapered such that diameter decreases rearward; an elastic member interposed between a rear end portion of the flange portion and a front-end portion of the taper cone fitted to the shank portion; and a stopper member disposed on the shank portion and adapted to support a rear-end face of the taper cone fitted to the shank portion. The outer circumferential surface of the taper cone is tapered at an angle equal to that at which the wall surface of the taper hole is tapered. The inner circumferential surface of the taper cone and the outer circumferential surface of the shank portion are tapered at least partially with respect to an axial direction such that diameter decreases rearward, at equal angles smaller than the angle at which the outer circumferential surface of the taper cone is tapered. The holder body is drawn rearward by means of a draw member disposed in the spindle. As a result, since the outer circumferential surface of the shank portion and the inner circumferential surface of the taper cone are tapered at equal angles, the taper cone is caused to radially expand to thereby press its outer circumferential surface against the wall surface of the taper hole. Accordingly, even when a force of drawing the holder body is weak, the outer circumferential surface of the taper cone is brought into close contact with the wall surface of the taper hole, so that the taper cone is sufficiently and strongly restrained within the taper hole, thereby enabling heavy-duty cutting and high-accuracy cutting by means of a tool held by a tool holder attached to the spindle.
The outer circumferential surface of the shank portion is locked into the taper cone through the so-called wedging effect, and the outer circumferential surface of the taper cone imposes a radially expanding force on the wall surface of the taper hole. As a result, an instantaneously-reached peak draw force can be maintained to thereby impose a large restraint force on the tool holder, thereby closely fitting the tool holder into the taper hole. The close fit of the tool holder into the taper hole yields excellent attenuation effect, thereby enabling high-speed cutting and low-speed, heavy-duty cutting.
According to the present invention, the inner circumferential surface of the taper cone and the outer circumferential surface of the shank portion are tapered substantially entirely with respect to the axial direction such that diameter decreases rearward, at equal angles smaller than the angle at which the outer circumferential surface of the taper cone is tapered. Thus, preferably, through effective use of the entire inner circumferential surface of the taper cone, the tool holder is used with a spindle in which a taper hole having a 1/10 taper is formed.
In another tool holder of the present invention, merely the outer circumferential surface of a front portion of the shank portion and the inner circumferential surface of a front portion of the taper cone are tapered at equal angles smaller than the angle at which the outer circumferential surface of the taper cone is tapered, thereby reducing an area to be tapered. Also, the outer circumferential surface of a rear portion of the shank portion and the inner circumferential surface of a rear portion of the taper cone are formed so as to extend straight. The rear portion of the taper cone is fitted to the rear portion of the shank portion. Thus, through use of the tool holder, cutting can be performed easily and accurately while vibration involved is comparatively small. Preferably, the tool holder is used with a spindle in which a taper hole having a 7/24 taper is formed.
In the tool holder of the present invention, a slit is preferably formed in the taper cone and is filled with an elastic member of, for example, fluorine-contained rubber, such that the elastic member is bonded to the taper cone. Accordingly, the filling elastic member prevents entry of dust.
In the tool holder of the present invention, the elastic member preferably comprises a plurality of Belleville springs and a washer disposed at at least the front or rear side of the plurality of Belleville springs, thereby applying a rearward force to the taper cone. Through modification of the number and thicknesses of washers, the position of the taper cone can be adjusted in the axial direction.
In the tool holder of the present invention, at least any one of a washer, a spring washer, and a nut to be screw-engaged with a rear-end portion of the shank portion is preferably disposed at the rear-end portion of the taper cone so as to support the rear-end portion of the taper cone, thereby enabling use of different pull studs.
In the tool holder of the present invention, preferably, a pair of arcuate shims are removably attached to a rear-end face of the flange portion and are adapted to abut the end face of the spindle, thereby enabling effective use of the holder body over a long period of time. Also, through use of shims of appropriate thickness, the gap between the end face of the spindle and the rear-end face of the flange portion can be eliminated.
In the tool holder of the present invention, components of the tool holder, such as the holder body and the taper cone, are preferably treated for corrosion protection, thereby enabling long-term use of the components.
To achieve the second object, the present invention provides a tool holder attachment mechanism comprising an annular spacer having a thickness corresponding to a specified gapxc2x1manufacturing tolerance (preferably, the sum of the specified gap and a maximum manufacturing tolerance) and being bonded to the end face of the spindle. Thus, a single spacer is applicable to every tool holder. The end face of the flange portion of the tool holder can be reliably brought into close contact with the end face of the spindle via the spacer. Further, the spacer assumes an annular shape so as not to fly out during high-speed rotation, thereby improving safety.
In the tool holder attachment mechanism of the present invention, the spacer is made of wear-resistant steel or hard rubber. Thus, even when the taper hole formed in the spindle wears, the spacer absorbs the wear to thereby maintain close contact between the taper shank portion and the taper hole, so that the tool holder can be reliably attached to the spindle.
In the tool holder attachment mechanism of the present invention, the adhesive application face of the spacer is preferably roughened so as to enhance bonding with the end face of the spindle, thereby establishing a strong bond between the spacer and the end face of the spindle.
In the tool holder attachment mechanism of the present invention, a portion of the spacer corresponding to a drive key projecting from the end face of the spindle is preferably cut out, so that the spacer can be bonded to the end face of the spindle without interference with the drive key.
In the tool holder attachment mechanism of the present invention, when a gap formed between the spindle and the flange portion of the tool holder upon attachment to the spindle becomes smaller than a gap specified in the industrial standard if the spacer is not present, the face of the spacer which faces the flange portion is preferably ground in an amount equal to the difference between the specified gap and the gap between the spindle and the flange portion, to thereby reduce the thickness of the spacer. Thus, the spacer bonded to the end face of the spindle permits use of a tool holder involving a problem in that a gap specified in the industrial standard cannot be formed. Further, through use of the spacer, close contact can be maintained between the taper shank portion and the taper hole, so that the tool holder can be reliably attached to the spindle.
In the tool holder attachment mechanism of the present invention, the spacer has a thinner portion around a cut-out portion. The thinner portion has a thickness less than a specified gapxc2x1manufacturing tolerance (preferably, than the specified gap less the manufacturing tolerance). Thus, even when a drive key projects from the end face of the spindle, the spacer bonded to the end face can be easily ground to a required thickness.